So, lets say that (and this is not from any experience from any campaign… really) that you are the mighty and terrible barbarian Ben the Bloodthirsty in an all evil campaign. Let’s say that some old priest of Pelor says, “The only thing to fear is fear itself” right before you smash his face in with your mace. Now, this is a predicament to you because BEN THE BLOODTHIRSTY FEARS NOTHING, and while you would much like to throttle fear with your bulging biceps, one of your party members reminds you that fear is just a concept with no puny neck for you to throttle.

So you decide to go for the next best thing and kill Bane instead, much to the annoyance of your DM who now has to come up with some convoluted quest to get a holy sword of Torm or something, to fight Bane at wherever he is, all while satisfying that one guy in the party who actually read the Forgotten Realms’ books and speaks up when things differ from cannon (I hate you Evan).

One of the most clichéd ways to kill a god (but one of the simplest), this is when you and your chums go after some sort of artifact (usually of a rival of the god your trying to kill) and kick some divine butt. It could also be a artifact that represents the gods power, which when destroyed either kills the god, or weakens them to the point where they can be fought and killed. Of course there’s always the question of… “Wait, if this artifact already existed for millennia, why didn’t someone use/destroy it already.” Now the simple answer would be “Shut up, Evan” but we’re a good DM so we’ll come up with excuses.

1. The weapon is really difficult to get.

Doesn’t explain why another god didn’t just swoop down and get it, so you’ll have to say “well, they can’t because that's just how god’s work”, or add a few more "really"s.

2. Killing a god has a price.

For example, the killer dies too. Or you become the new god of whatever it was the sucker you killed did. Cool if it’s Tyr or something… less cool if it’s Wastri.

3. It’s a new artifact that you need to create.

A bit better at defending, especially when you throw prophesy in the mix. If you can only mix the poison when the planets align and you have the blood of the chosen one... that's difficult to do at any other time.

If your gods are more wishy washy astral things fed off the beliefs of mortals, might I suggest killing their source of belief? While this could go into uncomfortable territory (as Ben tried to enact genocide onto believers of Bane) you could also pull it off not by killing believers, just making them lose faith in their deity. It’s also a nice touch as it allows you to start killing a deity from a lower level, as every argument/sacrilege is a step toward killing your celestial foe. Here are some ways to do so.

1. Destroying Proof

Yes this will lead you on a quest to destroy the god's artifact like above, but it’s different as you're not destroying the artifact to directly hurt the god, you're destroying it so it can’t be seen on earth, one less proof of the gods existence.

2. Portfolio Shredding

So gods are pretty much the logos of the stuff in their portfolios, like Pelor with the Sun, Lolth with Drow, Mickey Mouse with Disney. So all you need is to take that stuffed portfolio of things and dump them into another god and Presto! you have a god that stands for nothing... and likely doesn't get prayed much to.

3. Magic

Similar to "The Sword of Convenient Plot Purposes" this is just a spell that makes everyone forget the gods existence, slowly starving the god to death.

4. Arguing

This might be one of the most humorous and fun ways to kill a god. Spend the entire campaign going to pilgrimage sites of the god and arguing that they A. Don't exist, or B. They've been killed off. Either way, it would probably be an interesting campaign based on running away from monsters the supposedly non-existing deity sends at you.

5. Genocide

Pretty simple, kill all the followers. The least favored way to do it, in my humble opinion, as it leaves a bad taste in my mouth regardless if it's "just a game" (and I would know, I've been a cannibal in one campaign but even I have standards). However, it is an option so here it is, no believers, no belief, no god.

Probably, one of the more interesting ways to "kill" a god, beating them in a bet and winning their powers is possible.

According to sources (Evan and WIki) this is how Bane, Bhaal and Myrkle got their powers, by playing bowling with Jergal, god of death at the time (I can't make this up). Maybe you beat them in cards, or if your classy chess. Or maybe you play D and D with them, staring a new campaign with your characters from the previous campaign attaining godhood at the end... Wow... layers.

If you've always liked gods being laid back bros and brodettes, this is for you.